import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;


public class broken {
	
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new FileReader("broken.in"));
		for (;;) {
			int n = Integer.parseInt(br.readLine());
			if (n==0) break;
			
			char[] input = br.readLine().toCharArray();
			
			int x = 0, y=0;
			int max=0;
			HashMap<Character, Integer> hs = new HashMap<Character, Integer>();
			
			for (; y<input.length; y++) {
				char c = input[y];
				if (hs.size()<n) {
					int times = 1;
					if (hs.containsKey(c)) times = hs.get(c)+1;
					hs.put(c, times);
				} else {
					if (hs.containsKey(c)) {
						int times = hs.get(c) + 1;
						hs.put(c, times);
					} else {
						// entferne alles bis zum letzten vorkommen von input[x]
						int start = y;
						for ( ; start>=x; start--) {
							if (input[start]==input[x]) break;
						}
						for (int i=x; i<=start; i++) {
							int atimes = hs.get(input[i]) - 1;
							if (atimes==0) hs.remove(input[i]); else hs.put(input[i], atimes);
							x++;
						}
						hs.put(c, 1);
					}
				}
				if (max<y-x+1) max=y-x+1;
			}
			System.out.println(max);
		}
	
	}

}